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Here's the scenario. You pull your baby out of the garage and notice that something has leaked out of the rear of the car. The odd thing is that the leak is not dead center of the rear, it's more to the left (or right if you're really having a bad day). What you are probably experiencing is the failure of the output shaft seal (normally the driver's side in the C5). I say probably because doing a little "wide open throttle" (or WOT as it is known here) can also cause you to wake up to a wet spot. In the case of WOT, this is a normal occurrence, as the differential has a exhaust outlet in which differential fluid will leak out of in order to relieve case pressure. The professional way if the leaking source is not obvious is to add some Black Light Oil Dye to the differential and then check the area with a black light after some normal driving. The fluid will glow either a bright yellow or bright green. This allows you to identify the leak without doing unnecessary and costly repairs. If you want the GM dye, one place you can order it from the

You can clearly see how the leak is confined to about a 180 degree area of the seal. The differential side covers have been known to leak due to a design flaw although in this situation, this is not the case (just be aware of that possibility). We know that this is not the case here as oil does not leak in a upward direction.

Now that we have identified the leaking source, let's get that baby fixed!

The first thing you should do is get some replacement differential fluid. Now would be a good time to replace that old, stinky stuff. The service manual calls for SAE 75W-90 Synthetic Gear Lubricant or equivalent. The manual also states to add 4 ounces of Limited-Slip Differential Lubricant Additive. Today, you can buy this gear oil already mixed. Mobile one and Red Line are two brands that I found with the limited-slip additive already built in.

NOTE! After replacing the differential gear oil, do not go WOT for a minimum of 300 miles. Doing so may result in scoring of the ring and pinion gears leading to differential noise!!!

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